Heterojunction bipolar transistors (“HBTs”) have been in use for quite some time, particularly for use in signal amplifiers. The base regions of conventional HBTs were constructed using narrow band-gap materials. The use of these materials provided HBTs that worked well at high speeds, but were deficient in their ability to cope with high power applications. Accordingly, later conventional HBTs made use of type III-N materials in fabrication of the base region with wider band-gaps. While these III-N HBTs are capable of handling higher powers, they suffer many disadvantages. For example, it is difficult to make the base region sufficiently conductive, thus requiring very high activation energy to operate the HBT. Further, constraints placed on the base transit time and the base resistance continues to hold operating frequencies well below their theoretical maxima.
Increasing the conductivity in the base region may solve several disadvantages with conventional systems. For example higher p-type conductivity in the base region would lead to a reduction of the Early effect and enhance linearity. In order to achieve optimal performance for III-N HBTSs, it is highly desirable to exploit a more effective approach to increase the free hold concentration in the base to p>1019 cm−3, while simultaneously limiting the deleterious influence of neutral and ionized impurity scattering by Mg in the p-type materials. The minimization of base transit time and base resistance, however, represent conflicting design goals, and effective mitigation strategies have to date proven elusive.
Therefore, there is a desire for improved HBTs that address one of more of the disadvantages discussed above. Various embodiments of the present invention address these desires.